Immersion lens with magnetic shield for charged particle beam system

ABSTRACT

An immersion lens for a charged particle beam lithography system includes a magnetically floating shield that limits a deflection magnetic field from creating eddy currents in electrically conductive components of the system downstream from the shield. The surface of the shield lies parallel or approximately parallel to a magnetic equipotential surface of the focusing magnetic field so that the shield does not affect the focusing magnetic field. The shield is, e.g., a ferrite disk or a hollow ferrite cone defining a central bore for passage of the charged particle beam.

FIELD OF THE INVENTION

The present invention relates to charged particle beam lithography andmore particularly to immersion lenses for such lithography.

BACKGROUND

Lithography is a technique used to fabricate semiconductor devices andintegrated circuits. In lithography, a target substrate (usually a maskblank or semiconductor wafer) is coated with one or more layers ofphotoresist materials (resist). The resist is selectively exposed to aform of radiation, such as ultraviolet light, x-rays, electrons, andions. The resist is then developed to remove part of the resist. Theremaining part of the resist protects the underlying regions of thetarget. Regions from which the resist has been removed are subject tovarious additive (e.g., lift-off) or subtractive (e.g., etching)processes that transfer a pattern onto the target surface.

An electron beam or ion beam lithography system 110 (shown in FIG. 2)includes a charged particle (electron or ion) source 184 that generatesa charged particle beam 116 directed through aperture plates 118, ablanking deflector 121, and focusing lenses 120 before reaching a finalmagnetic lens 112. Lens 112 further directs beam 116 onto a target 159held on a target support 122 (also known as a stage). These lenses areelectromagnetic or electrostatic, not light optic, structures. Chargedparticle source 184 generates the electron or ion beam. A controlcomputer 123 controls the operation of lithography system 110.

One type of such lithography system is the variable axis immersion lenselectron beam system, see, for example, U.S. Pat. No. 4,544,846, toLangner et al. (herein “Langner et al.”), incorporated by reference inits entirety. FIG. 2 and FIG. 4A of Langner et al. are reproducedrespectively as FIG. 1A and FIG. 1B of the present disclosure. TheLangner et al. Background section describes a variable axis electronbeam projection system as being one where the electron optical axis ofthe projection system is shifted so as to be coincident with a deflectedelectron beam used to write on the target at all times. Shifting theelectron optical axis is said to cause the electron beam to always landperpendicular to the target and to eliminate lens aberrations which arecaused by off-axis electron beams.

The variable axis immersion lens electron beam system includesdeflection coils 43 and 45 (see FIG. 1A that depicts this structure incross-section) that deflect an electron beam (shift the axis of theelectron beam) so as to direct the beam to the desired location on thetarget 59. An immersion lens 12 includes one or more excitation coils 41and 53 that generate a magnetic field when conducting an electriccurrent (also called the focusing magnetic field). The focusing magneticfield has magnetic field lines that extend from a pole piece 13 to apole piece 14 (FIG. 1B). The focusing magnetic field thus immerses atarget 59 in an approximately uniform magnetic field (hence the nameimmersion lens) where the magnetic field strength is maximum near thesurface of pole piece 14.

A deflection coil 11 generates a magnetic field (also called thedeflection magnetic field) that shifts the magnetic axis of immersionlens 12 (hence the name variable axis) to coincide with the shifted axisof the electron beam. Deflection coils 11, 43, 45 vary the deflectionmagnetic field over time as the axis of the electron beam is shifted toscan target 59 during lithographic processes.

The varying deflection magnetic field creates eddy currents inelectrically conductive system components downstream from deflectioncoil 11 (with respect to the direction of propagation of the electronbeam), such as a target (wafer or mask blank) holder 16, a holderhandler 20, and pole piece 14. Additionally, the varying deflectionmagnetic field may create eddy currents in a target 59 that is, e.g., asemiconductor wafer. The eddy currents in the above-described elementsgenerate opposing deflection fields that deflect the electron beam,thereby creating placement error of the electron beam.

Accordingly, a disadvantage of the variable axis immersion lens isplacement error caused by eddy currents generated by the deflectionmagnetic field. Alternatively, the system components subject to thedeflection magnetic field can be of non-electrically conductivematerials. However, the cost of the system increases with use of suchmaterials. Thus, what is needed is a method and an apparatus thatprevent the deflection magnetic field from radiating into electricallyconductive components of the system downstream from the deflection coil,without adversely affecting the focusing magnetic field.

SUMMARY

In one embodiment, an immersion lens for a charged particle beam systemincludes a magnetically floating ferrite disk that shields non-magneticbut electrically conductive components of the system from the timevarying magnetic field generated by the deflection coil while notdisturbing the static magnetic field used for beam focusing. (Floatinghere means not forming a part of a magnetic circuit.) The disk ismounted downstream from the deflection coil (with respect to thedirection of propagation of the charged particle beam) such that asurface of the disk is approximately parallel to a magneticequipotential surface of the magnetic field (also called the focusingmagnetic field) generated by the immersion lens. The disk limits thedeflection magnetic field from radiating into the electricallyconductive system components downstream from the disk.

In another embodiment, an immersion lens for a charged particle beamsystem includes a somewhat similar magnetically floating ferrite conethat shields electrically conductive elements from the deflectionmagnetic field. The cone is similarly mounted downstream from thedeflection coil such that the surface of the cone is parallel orapproximately parallel to a magnetic equipotential surface of thefocusing magnetic field. The cone limits the deflection magnetic fieldfrom radiating into the electrically conductive system componentsdownstream from the cone.

Various embodiments will be more fully understood in light of thefollowing detailed description taken together with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a prior art variable axis immersion lens.

FIG. 1B illustrates a focusing magnetic field generated by the prior artvariable axis immersion lens of FIG. 1A.

FIG. 2 illustrates a variable axis immersion lens lithography system inaccordance with one embodiment.

FIG. 3A illustrates an immersion lens with a shielding disk inaccordance with one embodiment.

FIG. 3B illustrates the disk of FIG. 3A.

FIG. 3C illustrates the magnetic field generated by the immersion lensof FIG. 3A.

FIG. 3D illustrates the magnetic field generated by the deflection coilof FIG. 3A.

FIG. 4A illustrates an immersion lens with a shielding cone inaccordance with one embodiment.

FIG. 4B illustrates the cone of FIG. 3A.

FIG. 4C illustrates the magnetic field generated by the immersion lensof FIG. 4A.

FIG. 4D illustrates the magnetic field generated by the deflection coilof FIG. 4A.

Use of the same reference numbers in different figures indicates thesame or like elements.

DETAILED DESCRIPTION

In one embodiment, an otherwise conventional charged particle beamlithography system 110 (shown in a side view in FIG. 2) includes amagnetic field shield such as a magnetically “floating” disk 160 (shownby itself in perspective view in FIG. 3B) or a magnetically “floating”cone 162 (shown by itself in perspective view in FIG. 4B). The magneticfield shield limits magnetic field 172 (FIG. 3D and FIG. 4D) generatedby deflector coil 111 from radiating downstream from the magnetic fieldshield. The magnetic field shield does not affect magnetic field 170(FIG. 3C and FIG. 4C) generated by excitation coil 141 because themagnetic field shield is mounted so its upper surface is parallel (orapproximately parallel) to an equipotential magnetic surface 161 ofmagnetic field 170.

In one embodiment, lithography system 110 includes charged particle(e.g., electron) source 184, aperture plates 118, blanking deflector121, focusing lenses 120, an immersion lens 112, stage 122, and controlcomputer 123. These are all conventional. Additional conventionalstructures, such as mechanical supports, mounting hardware, cooling,electrical, and vacuum elements (including the enclosure), are not shownfor the sake of clarity but are understood by one skilled in the art tobe present in lithography system 110.

In one embodiment, immersion lens 112 (see FIG. 3A that depicts thisstructure in greater detail in cross-section) includes an excitationcoil 141 that generates magnetic field 170 (also called the focusingmagnetic field) represented by magnetic field lines that extend from apole piece 134 to the pole piece 114 (FIG. 3C). Pole piece 114 is partof a magnetic lens circuit that includes inner pole piece 134, typicallyof ferrite, an outer typically iron pole piece 150, and an outertypically iron return yoke 140. Pole piece 114, return yoke 140, andpole piece 150 are collectively referred to as an “iron pole piece” butare not necessarily of iron.

Pole piece 134 is separated from outer pole piece 150 by a non-magneticspacer 154 (shown in FIG. 3A and FIG. 4A). Spacer 154 is of materialsuch as Vespel® from DuPont of Wilmington, Del. Alternatively, polepiece 134 is separated from outer pole piece 150 by an air gap 156(shown in FIGS. 3C, 3D, 4C, and 4D). A pair of deflector coils 143 and145 deflect a charged particle beam (shift the axis of the chargedparticle beam) to scan a target 159. Deflector coil 111 generatesmagnetic field 172 (also called the deflection magnetic field) tocoincide the magnetic axis of magnetic field 170 with the shifted axisof the charged particle beam.

In one embodiment, a relatively thin and magnetically floating disk 160(e.g., of ferrite) is mounted downstream from deflector coil 111. Disk160 is mounted so that its upper surface 163 (FIG. 3C) is approximatelyparallel to a magnetic equipotential surface 161 of focusing magneticfield 170, e.g., where the magnetic field lines are approximatelyperpendicular to surface 163. So mounted, disk 160 has little influenceon focusing magnetic field 170 because disk 160 is thin, orientedperpendicular to field lines along magnetic equipotential surface 161 offocusing magnetic field 170, and magnetically floating, e.g., notmagnetically coupled to the magnetic circuit including pole piece 134,pole piece 150, outer return yoke 140, and pole piece 114. In oneimplementation, disk 160 is mounted to pole piece 134 by a non-magneticand non-conducting mount 164 (for clarity, shown only in FIG. 3A) sothat it is magnetically floating. Mount 164 is of material such ascoated ceramic with a relatively high resistivity, in such a way thatthe disk's surfaces are electrically grounded but not subject to eddycurrents. Mount 164 is, for example, adhesively bonded at both endsusing conductive epoxy to mount disk 160 on pole piece 134.

In this embodiment, disk 160 prevents deflection magnetic field 172 fromradiating downstream from disk 160 (FIG. 3D). Deflection magnetic field172 conventionally has both lateral and azimuthal components, forming adipole with magnetic field lines that return in a loop to the oppositeside of deflector coil 111. Disk 160 shunts the lateral and azimuthalcomponents of deflection magnetic field 172 within its material, therebyclosing the loop of the magnetic field lines above disk 160. Lens fluxlines perpendicular to disk 160 pass directly through it. Therefore,disk 160 limits deflection magnetic field 172 from radiating downstreamfrom disk 160 without substantially influencing the focusing field.

In this embodiment, conventional system components are locateddownstream of disk 160, so that a backscatter electron detector 168,substrate 157, stage 122, and a stage drive 124 may be of non-magneticbut electrically conductive material (such as various metals) to reducetheir cost. The region downstream from disk 160 including detector 168,stage 122, and stage handler 124 is hereinafter referred to as the stageregion.

In one implementation, disk 160 defines a central opening (bore) ofradius r and is mounted so lower surface 165 of disk 160 isapproximately a distance 2r above upper surface 157 of target 159.Radius r is chosen so that the bore diameter exceeds the scanning areaof immersion lens 112 where the electron beam can be deflected. Disk 160has an overall radius of R. In one variation, overall radius R of disk160 is similar to the outer radius R of pole piece 134. Disk 160 has athickness t, and in one variation, t is about 3 mm. Despite being thin,disk 160 is not saturated by deflection magnetic field 172 becausedeflection magnetic field 172 is weak.

In one embodiment, instead of a disk 160, a magnetically floating andhollow cone 162 (FIG. 4A and FIG. 4B) is mounted beneath deflector coil111 as the magnetic field shield. Cone 162 (typically also of ferrite)functions similarly to disk 160. Cone 162 is located so its uppersurface 167 is parallel or approximately parallel to magneticequipotential surface 161 (FIG. 4C) of focusing magnetic field 170. Theshape of cone 162 allows its upper surface 167 to conform toequipotential surface 161 better than does surface 163 of disk 160.Similar to disk 160, cone 162 prevents deflection magnetic field 172from creating eddy currents in non-magnetic but electrically conductivesystem components downstream from cone 162 (FIG. 4D). In oneimplementation, cone 162 is mounted to pole piece 134 by mount 164 (forclarity, shown only in FIG. 4A) so that it is magnetically floating.

In one implementation, cone 162 defines a bore of radius r and ismounted so a lower surface 174 of the frustum is approximately adistance 2r above surface 157 of target 159. Radius r is chosen so thatthe bore diameter exceeds the scanning area of immersion lens 112 wherethe electron beam can be deflected. Cone 162 has an overall radius of Rand a height of H. In one variation, overall radius R of cone 162 ischosen as similar to the outer radius R of pole piece 134.

In one implementation, height H is chosen so that upper surface 167 ofcone 162 is parallel or approximately parallel to magnetic equipotentialsurface 161 of focusing magnetic field 170. Magnetic equipotentialsurface 161 can be determined with a magnetic probe, such as a Halleffect gaussmeter made by FW Bell & Co. of Orlando, Fla. Alternatively,computer modeling with a computer program such as “Optics” by Mebs Ltd.,of London, England, can be used to determine magnetic equipotentialsurface 161. After magnetic equipotential surface 161 is determined, aheight H is selected in accordance to overall radius R so that theresulting upper surface 167 of cone 162 has a minimum effect uponfocusing magnetic field 170.

Cone 162 has a thickness of t; in one variation, t is about 3 mm.Despite being thin, cone 162 does not become saturated by deflectionmagnetic field 172 because deflection magnetic field 172 is weak.

Although embodiments of the present invention have been described inconsiderable detail with reference to certain versions thereof, otherversions are possible. For example, mount 164 may couple disk 160 orcone 162 to pole piece 150 instead of pole piece 134. Alternatively,disk 160 and cone 162 are mounted to other convenient support structuresin lithography system 110. Therefore, the spirit and scope of theappended claims should not be limited to the description of the versionsdepicted in the figures.

What is claimed is:
 1. A variable axis immersion lens assembly for usewith a charged particle beam, where the electron optical axis of thecharged particle beam is shifted to be coincident at all times with adownstream deflected electron beam emanating from the immersion lens,said variable axis immersion lens comprising: at least two optical axisdeflection coils located coaxial to the charged particle beam, whichoptical axis deflection coils are used to shift the optical axis of thecharged particle beam to be coincident with a downstream deflectedelectron beam; an excitation coil located coaxial with respect to theoptical axis deflection coils; a magnetic field deflector coil used tocoincide a magnetic axis of a magnetic field generated by the excitationcoil with the shifted optical axis of the charged particle beam; a firstpole piece located coaxial to the excitation coil, the first pole pieceextending at least partly around the excitation coil; a magneticallyfloating field shield located coaxial to and downstream from themagnetic field deflection coil with regard to propagation of the chargedparticle beam from the magnetic field deflection coil; a support for atarget of the charged particle beam, which support is downstream withregard to propagation of the deflected charged particle beam from themagnetic field shield, wherein the magnetic field shield is locatedintermediate the magnetic field deflection coil and the support, therebylimiting a magnetic field generated by the magnetic field deflectioncoil from radiating downstream into areas protected by the magneticfield shield.
 2. The immersion lens assembly of claim 1, wherein thefirst pole piece is of iron.
 3. The immersion lens assembly of claim 1,further comprising a second pole piece located coaxial to the magneticfield deflection coil, the second pole piece extending at least partlyaround the magnetic field deflection coil.
 4. The immersion lensassembly of claim 3, wherein the second pole piece is of ferrite.
 5. Theimmersion lens assembly of claim 1, wherein the magnetically floatingfield shield is at least approximately parallel to a magneticequipotential surface of a magnetic field generated within the immersionlens by the excitation coil.
 6. The immersion lens assembly of claim 1,wherein the magnetically floating field shield is of ferrite.
 7. Theimmersion lens assembly of claim 1, further comprising a detectorlocated intermediate the magnetically floating field shield and thesupport for the target.
 8. The immersion lens assembly of claim 1,wherein the support for the target is of non-magnetic and electricallyconductive material.